1. Field of the Invention
The present invention relates to a cathode ray tube and, more particularly, to a cathode ray tube having an antistatic function.
2. Description of the Related Art
Since the outer surface of a front panel of an envelope of a cathode ray tube has a high surface electric resistance, a static charge accumulates during an operation of the cathode ray tube. This causes inconveniences. For example, dust adheres on the outer surface or an electrical shock is inflicted on a person.
In order to eliminate such inconveniences, Japanese Patent Disclosure No. 61-16452 discloses a cathode ray tube having a transparent thin film composed of a silicate and an inorganic metallic compound of a metal such as platinum, palladium, tin and gold, and formed on the outer surface of its front panel. However, since the silicate in the thin film is not conductive, the surface electric resistance of the thin film cannot be sufficiently decreased. Therefore a sufficient antistatic effect cannot be obtained.
According to the tests performed by the present inventors, when an inorganic metallic compound was not hygroscopic, a thin film was not conductive. When a hygroscopic inorganic metallic compound such as PdCl.sub.2 was added, a low conductivity was obtained. However, when the content of the compound was increased so as to obtain sufficient conductivity, the strength and optical characteristics of the thin film were greatly degraded.
In order to increase the strength of a reflection-free thin film formed on the outer surface of a front panel of an envelope of a cathode ray tube, Japanese Patent Disclosure No. 61-45545 proposes to add an oxide or a hydroxide of a metal such as Ti, Al, Mg, Ca, Zr, Na, and K in the film. However, since it is considered that metal atoms in the oxide or hydroxide are present where Si--O--Si bonds of the skeleton structure of a film-forming material such as SiO.sub.2, are fragmented as in FIG. 1, the oxide or hydroxide cannot obtain hygroscopicity. Therefore, the resultant film cannot be used as a film required to have conductivity.